Hollow workpieces having a thin walled cylindrical or tubular wall surface to be machined are easily distortable, and may be out of round and out of concentricity in their as-formed condition, prior to machining. A chuck has been developed for holding and gripping such workpieces to enable the circumference of the peripheral wall to be machined to an in-round and concentricity condition. Such a chuck is disclosed in the present assignee's U.S. Pat. No. 4,953,877 to Slachta et al, granted Sep. 4, 1990, and employs a plurality of discrete flexible chucking fingers actuated by a pliable inflatable ring for radially deflecting the fingers conjointly into engagement with the peripheral wall of the workpiece. With some workpieces, where the workpiece includes a rigid hub wall which is centered with respect to the machine spindle, it has been found necessary to grip a non-concentric wall of the part eccentrically, and a chuck of the type disclosed in the patent has been improved to achieve the result desired.
By way of example, for internal chucking applications, a workpiece may have a central, cylindrical, pre-machined rigid hub portion to which a thin, readily distortable stamped metal portion having a non-concentric outer peripheral wall to be gripped and machined is secured. It is necessary that the hub of the workpiece be centered in the chucking apparatus irrespective of the relative eccentricity of the wall of the stamped peripheral portion which is gripped. This assures that the wall of the stamped portion of the workpiece, even though it is gripped eccentrically, is machined concentrically with the pre-machined hub portion. The chuck may be used to grip the part in the same way when it is desired to machine the hub.